The pressure swing adsorption (PSA) process is a well known means for separating and purifying a less readily adsorbable gas component contained in a feedstream from a more readily adsorbable second component.
Pressure swing adsorption systems generally involve passage of the feedstream through equipment comprising two or more adsorbers containing beds of molecular sieves or other adsorbents which selectively adsorb the heavier components of the feedstream. The adsorbers are arranged to operate in sequence with suitable lines, valves, timers and the like so there is established an adsorption period during which the heavier components of the feedstream are adsorbed on the molecular sieve or other adsorbent and a regeneration period during which the heavier components are desorbed and purged from the adsorbent to regenerate it for reuse.
Such selective adsorption commonly occurs in the adsorption beds at an upper adsorption pressure, with the more selectively adsorbable component thereafter being desorbed by pressure reduction to lower desorption pressure. The beds can be purged at such lower pressures for further feedstream purification.
Such PSA processing is disclosed in U.S. Pat. No. 3,430,418 to Wagner and in U.S. Pat. No. 3,986,849 to Fuderer et al., wherein cycles based on the use of multi-bed systems are described in detail. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA system. Such cycles are commonly based on the release of void space gas from the product end of each bed in one or more cocurrent depressurization steps upon completion of the adsorption step. In these cycles, the released gas typically is employed for pressure equalization and for subsequent purge steps. The bed is thereafter countercurrently depressurized and often purged to desorb the more selectively adsorbed component of the feedstream from the adsorbent and to remove such gas from the feed end of the bed prior to the repressurization thereof to the adsorption pressure.
PSA processes were first used for gas separation in which only one of the key components was recovered at high purity. For example, from 100 moles of feedstream containing 80 moles hydrogen and 20 moles carbon monoxide, the process of Wagner, U.S. Pat. No. 3,430,418, could separate 60 moles of hydrogen at 99.999% purity, but no pure carbon monoxide could be recovered; 20 moles of carbon monoxide and 20 moles of hydrogen remained mixed at 50% purity each. A complete separation could not be made. Only the less adsorbable, light component was recovered at high purity.
For the recovery of a pure, more readily adsorbed heavy component, an additional step was necessary, namely, rinsing of the bed with a heavy component to displace the light component from the bed prior to depressurization. This rinsing step is described in several earlier patents. The problems with these processes are the following: (a) if the rinsing is complete and the light component is completely displaced from the bed, pure heavy component can be obtained, but the adsorption front of the heavy component breaks through to the light component and the latter cannot be recovered at high purity; (b) if the displacement of the light component is incomplete and if such bed is depressurized countercurrently to recover the heavy key component at the feed end, the light component still present in the bed reaches the feed end very rapidly and the purity of the heavy component drops. It is, therefore, not practical with the prior art processes to obtain both key components at high purity in a single PSA unit.
Such complete separations can be obtained, however, by two separate pressure swing adsorption processing units wherein each unit includes several fixed beds. From a feedstream containing, for example, hydrogen and carbon monoxide (CO), the first unit recovers pure hydrogen and a carbon monoxide-rich gas containing 70% carbon monoxide. This feedstream is compressed and passed through a second PSA unit which recovers pure carbon monoxide and a hydrogen-rich gas. The hydrogen-rich gas can be added as feedstream to the first PSA unit and then the cycle is repeated. The combination of the two independent PSA units can make an excellent separation at very high flexibility. For example, from a feedstream with two components this system can recover more than 99.8% of the adsorbable "light" component such as hydrogen at a purity of 99.999% and also recover essentially 100% of the more readily adsorbed, heavy component, such as carbon monoxide, at a purity higher than 99.5%.
A PSA process suitable for the recovery of both the less and more readily adsorbable components is described in British Patent 1,536,995 to Benkmann. The process is based on two beds in series cycle as shown in FIG. 2 of Benkmann. The feedstream is introduced to the lower bed which retains the more readily adsorbable component. The feed step is followed by a cocurrent depressurization step wherein the less readily adsorbable component is recovered from the upper bed and a copurge step in which the less readily adsorbable or light component is further displaced in the lower bed by a recycled stream of heavy components, so that the lower bed at the end of the step contains only the heavy component. At this moment, the connection between the upper and lower beds is interrupted by an automatic valve and the heavy product is recovered from the lower bed by (countercurrent) depressurization. The upper bed is in the meantime, also depressurized and purged to remove all of the heavy component. The step sequence of the upper and lower bed are interlocked and cannot be run with independent cycles. When additional intermediately adsorbable components are present in the feedstream, the Benkmann patent discloses continued copurging to displace the intermediately adsorbable components and pass them through both the lower and upper beds in a chromatographic fashion. Such a flow scheme allows the intermediately adsorbable components to be adsorbed in the effluent end of the upper adsorber. Since the presence of more readily adsorbable components in the effluent end of an adsorber bed can result in a purity reduction of the less readily adsorbable component, such flow schemes are generally avoided by those skilled in the art. Moreover, the step sequence of the upper and lower beds are interlocked and cannot be run with independent cycles. The flexibility of this system is, therefore, reduced. For example, since the upper and lower beds are interlocked in series, if process conditions such as feedstream composition change, it is at least inconvenient if not impossible to change the volume or time ratio of the two beds.
U.S. Pat. No. 4,171,207, to Sircar, discloses a process wherein multicomponent feedstreams containing hydrogen as a primary component, a secondary key component that is more readily adsorbed by the adsorbent than hydrogen and one or more dilute components more readily adsorbed than both hydrogen and the secondary key component, are subjected to adsorption in a PSA system for the separate recovery of high purity hydrogen and of the secondary component. The process employs a plurality of trains of adsorber beds wherein each train of adsorbent beds undergoes a cycle having the following sequence of steps: (a) adsorption at a superatmospheric pressure between two beds in series, (b) high pressure rinsing, i.e., copurging, of the second bed, (c) depressurization of the second bed to substantially ambient pressure and recycling a portion thereof as said high pressure rinse, (d) evacuation of the second bed to subatmospheric pressure, (e) pressure equalization of the first bed, (f) depressurization of the first bed to substantially ambient pressure, (g) purging of the first bed with a portion of the product from step (c) and repressurization of both beds.
The process of above-identified U.S. Pat. No. 4,171,207 discloses that a portion of the secondary component depressurization product is used for purging the first bed and that the remaining portion is used for the high pressure rinsing. In order to recover the secondary component from the second adsorber, the patent discloses an evacuation step, i.e., step (d). Such evacuation steps are often commercially undesirable due to the large diameter piping, compressors and costs associated therewith as well as perceived notions of unreliability with vacuum systems.
U.S. Pat. No. 4,512,780, to Fuderer, discloses a PSA process that utilizes at least four adsorber beds for the separation of a feedstream containing less readily adsorbable component, an intermediately adsorbable component and a more readily adsorbable component, by employing a cocurrent displacement step in which the less readily adsorbable component is essentially completely removed from the adsorption bed. The bed is then cocurrently depressurized with the intermediately adsorbable component being discharged from the product end thereof as a product of desired purity. The displacement gas is obtained by diverting a portion of the gas released from that or another bed in the system during the cocurrent depressurization or the countercurrent depressurization steps. The process of the above-described patent avoids the problems associated with interlocked upper and lower adsorber beds in order to provide improved operating flexibility. Moreover, it does not require a vacuum step, yet it provides a high purity intermediately adsorbable product at high recoveries. However, since the intermediately adsorbable product is obtained from the product end of the adsorber, and hence adsorbed therein, a reduction in the purity of the less readily adsorbable component can occur.
U.S. Pat. No. 4,813,980, to Sircar, discloses a process directed to the recovery of ammonia synthesis gas, i.e., H.sub.2 and N.sub.2 as the less readily adsorbable component, and CO.sub.2, i.e., more readily adsorbable component, from an oxidized hydrocarbon reformate. The process of the above-identified U.S. patent utilizes two groups of adsorber beds connected in series. The adsorbers in the first group undergo a cycle sequence comprising the following steps, (a) adsorption, (b) high pressure rinse, i.e., copurge, (c) depressure, (d) evacuation, i.e., vacuum, (e) equalize pressure, and (f) final pressurization. The adsorbers in the second group undergo a cycle sequence comprising the following steps (1) adsorption, (2) pressure equalization, (3) depressurizing, (4) purge, and (5) repressurization. The process uses vacuum steps and does not provide for the recovery of high purity intermediately adsorbable components.
Another process that utilizes similar PSA cycle sequences as disclosed in the above-identified U.S. Pat. No. 4,813,980, but provides for the recovery of a primary key component, a secondary key component that is more readily adsorbed than the primary key component, and minor dilute tertiary components that are less readily adsorbed than the secondary key component, is disclosed in U.S. Pat. No. 4,790,858, to Sircar. This patent, however, discloses the use of vacuum steps and three sets of adsorption columns to achieve the desired separation.
U.S. Pat. No. 4,846,851, to Guro, et al., relates to a process for purifying ammonia systhesis gas and discloses a PSA process having two adsorber sections in series. The patent does not specifically disclose the use of vacuum desorption, but rather discloses high pressure rinsing, i.e., copurging, of the first adsorber section with recycle of the effluent stream to the first adsorber section feed. As such, the process can provide a purified product from the first adsorber section, however, additional adsorbent inventory can be required to accomodate the recycle. Moreover, the process does not provide a purified intermediately adsorbable product.
Additionally, the purge gas used for the low pressure rinsing of the first adsorber section of the process disclosed in above-identified U.S. Pat. No. 4,846,851 is provided from the desorption or purge products from the second adsorber section. In fact, it is stated at col. 5, lines 17 to 19 that, "An important feature of the present invention is in the use of gases desorbed from the B bed in the regeneration of the A bed." Such use of desorbed, i.e., adsorbable gases is often undesirable because the adsorbable gases can be adsorbed in the effluent end of the adsorber, thus leading to contamination of the adsorption effluent stream. Morover, the adsorbable components present in the purge can occupy adsorption sites in the adsorbent thereby reducing the adsorptive capacity of the first adsorber section.
European Patent Application No. 88310723.7, Publication No. EP 0 317 235 A2, published May 24, 1989, discloses a reforming process to produce hydrogen, carbon monoxide, carbon dioxide and water and a PSA separation process that can be used to recover carbon monoxide from mixtures with hydrogen and carbon dioxide, i.e., the carbon monoxide is the intermediately adsorbable component. The reference discloses the use of two arrays of adsorber vessels in series operated such that the hydrogen is obtained from the effluent end of the second vessel, the carbon dioxide is obtained from the feed end of the first vessel and the carbon monoxide is obtained from both the effluent end of the first vessel and the feed end of the second vessel.
Hence it can be seen in view of the forgoing that separating a feedstream by PSA processes to provide a high purity less readily adsorbable component, e.g., hydrogen, and an intermediately adsorbable component, e.g. carbon monoxide, when more readily adsorbable components are present has proved to be difficult. Accordingly, PSA processes are sought which can provide a high purity intermediately adsorbable component without requiring vacuum desorption and without using adsorbable gases for low pressure purging.
Moreover, such improved PSA processes are particularly desired in processes for the production of hydrogen and carbon monoxide from the reforming of methanol. Methanol reforming and related product separation processes are disclosed in U.S. Pat. No. 4,692,322, to Moller, et al., and 4,316,880, to Jockel, et al.